1. Field of the Invention
The present invention relates to a semiconductor memory device including electrically programmable irreversible memory elements.
2. Description of the Related Art
A semiconductor integrated circuit absolutely requires non-volatile OTP (One Time Programmable) memories that can continuously hold stored information after power off. They have been widely employed for redundancy use in massive memories such as DRAM and SRAM, for tuning use in analogue circuits, for code storage use in encryption keys, and for use of storing production histories.
Conventional memory redundancy utilizes a ROM that uses laser fuses (see JP-A 2001-168196). The laser fuse ROM includes an ordinary wire, which is blown out with laser light to store information. Therefore, it requires no particular processes to form laser fuses and has been considered a cheapest non-volatile memory. The laser fuse ROM, however, requires a particular fuse blower and the blowing step using the same and accordingly has a weakness associated with a higher programming cost. In addition, the minimum dimension of the laser fuse is determined by the wavelength of the laser light used. Accordingly, it can not keep pace with fine patterning of other circuit parts and causes a problem associated with the share of the area that gradually increases. Further, as programming is executed using laser, programming can not be executed except for only in a wafer state and can not be used to relieve a failure found in a fast test after packaging. Therefore, using no laser blower increases the expectation of application to an electrically programmable non-volatile OTP memory.
There has been an electrically programmable non-volatile OTP memory that uses an antifuse. The antifuse is a MOSFET including a thin gate oxide, to which gate oxide a high voltage is applied to break down the insulator to store information. The programming mechanism for the antifuse is extremely simple because it is sufficient to place a large potential difference between both ends of the antifuse on programming data. Thus, the antifuse is a memory element that is most expected for use in the non-volatile OTP memory in the future.
There is a problem, however, to be solved on boosting the write voltage. The antifuse has one end connected to a write power source, and the other end connected to a write and a read circuit. Therefore, on boosting the write voltage, only the voltage on the one end is boosted while the other end still remains at a low voltage. As a result, a high voltage is applied across the gate oxide of the antifuse to break down the insulator and cause false write possibly.